1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an optical film having a property of a wide viewing angle by compensating a phase difference and a liquid crystal display device having the optical film.
2. Discussion of the Related Art
Recently, to meet the needs of the times, a flat panel display device having small size, lightweight, and low power consumption is a subject of research. Accordingly, a thin film transistor-liquid crystal display (TFT-LCD) device that has high color quality and small size is developed. A conventional liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal (LC) molecules. The LC molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment direction of the LC molecules can be controlled by applying an electric field to the LC molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the LC molecules also changes. Since the incident light is refracted to the orientation of the LC molecules due to the optical anisotropy of the aligned LC molecules, images are displayed.
In the LCD device of the above-mentioned structure, research is being performed to improve the viewing angle and the contrast ratio. Especially, modes to perform wide viewing angle are represented by vertical alignment (VA) mode, in-plane switching (IPS) mode and electrically controlled birefringence (ECB) mode.
In the ECB mode, a liquid crystal layer uniformly oriented is interposed between orthogonal polarizers and transmittance is changed according to the birefringence effect by the applied voltage. A conventional LCD device of optically compensated birefringence (OCB) mode, which is one of ECB modes, has a symmetric bend structure such that an angle between the long axis of LC molecules and the substrate is nearly 90° (degrees) toward the midway point between the substrates, and then the angle is gradually decreased as approaching to the substrates. Therefore, the conventional LCD device of OCB mode has low response time.
FIG. 1 is a schematic perspective view of a liquid crystal display device of a conventional mono-domain optically compensated birefringence mode.
In the OCB cell 12, after rubbing in the same direction on the inner surfaces of the upper and lower substrates 10 and 11, a bend structure is formed by applying a voltage. Since the LC molecules rotate fast when a voltage is applied, a time for realignment, i.e., a response time is less than about 5 milliseconds. Therefore, the OCB cell has a good response property so that residual images cannot remain and the moving images can be well displayed. Even though the LCD device of a conventional mono-domain OCB mode has a good response property, the viewing angle is relatively narrow. To improve the viewing angle property, a biaxial film 15 is interposed between a first polarizing plate 13 and the upper substrate 10.
FIG. 2 is a schematic view of a compensation film and a liquid crystal cell of a conventional mono-domain optically compensated birefringence mode illustrating a compensation of a phase difference of the liquid crystal layer by a biaxial film.
In the bend structure of the OCB cell 20, the LC layer is aligned parallel to the substrates adjacent to the substrates and perpendicular to the substrate at the middle of the substrates. In the multi-domain structure, even though the alignment direction of each domain is different from each other, the alignment directions at the middle of the LC layer are same, i.e., vertical. Therefore, the compensation for the LC cell of this structure can be done along two directions by single compensation film, i.e., a conventional biaxial film 30. However, the conventional biaxial film cannot effectively compensate the birefringence of the LC cell of the bend structure so that the viewing angle of the LC cell is less than 100° (degree) and the LC cell has a limit in realization of a wide viewing angle. To improve this problem, the compensation film of a discotic liquid crystal having a disc-like molecular structure is suggested.
FIG. 3 is a schematic view of a compensation film of discotic liquid crystal and a liquid crystal cell of a conventional mono-domain optically compensated birefringence mode.
In FIG. 3, first and second compensation films 32 and 34 of discotic LC are respectively disposed on the upper and lower sides of the LC cell so that the birefringence of the nearly horizontally aligned LC 35 and the vertically aligned LC 37, can be compensated.
FIG. 4 is a schematic view of a compensation film of discotic liquid crystal.
In FIG. 4, the compensation film of a discotic LC is formed by splayed alignment of discotic LC 40 on the tri acetyl cellulose (TAC) 38.
FIG. 5 is a schematic view illustrating a compensation principle of a compensation film of discotic liquid crystal.
In FIG. 5, when the light passes the LC cell 36, where some of light passes normally and other light passes obliquely, since the phase of transmitted light differs according to the incident angle, the optical property of the transmitted light differs according to the viewing angle. The phase difference of the light transmitted through the LC cell can be expressed as a retardation dΔn (d.delta.n) that is defined by multiplying the thickness of the medium that the light passes through by the difference of refractive indices in the plane normal to the direction of light propagation ne-no. To compensate the phase difference of the LC cell, a compensation film 40 of LC, whose retardation is same in absolute value but negative in sign with the LC cell, is used so that the viewing angle of the LC cell of OCB mode can be increased.
However, the compensation film of discotic LC has several drawbacks. The types of discotic LC are too few, and splayed alignment of the discotic LC is hard to form. Moreover, since it is difficult to increase the thickness of the discotic LC layer, the compensation film of discotic LC is not adequate for LCD devices of various modes. Especially, for the LCD device of OCB mode different from twisted nematic (TN) mode, since the change of the specification of the LC layer is frequently required, the compensation film of discotic LC is hard to be adopted. Furthermore, the use of the compensation film of discotic LC reduces the brightness of the LCD device by about 30% to 40%.